Fiber optic systems are used in high data rate local area network applications because of the extremely wide bandwidth that is available. Current LANS are characterized by multiple users who require simultaneous transmission of large bandwidth data, such as in video applications. The development of high frequency lasers with bandwidths of 10 GHz or more has made high data rate, multi-channel transmission systems possible. Two multiplex transmission schemes, frequency division multiplexing (FDM) and time division multiplexing (TDM), are practical approaches to utilizing the large bandwidth provided by fiber optics.
FDM is the preferred technique when system flexibility is a consideration. FDM allows a mixed transmission of analog and digitally modulated carriers, whereas TDM is only used in digital systems. Furthermore, transmission channels can be more easily added or deleted with FDM. Adding a new channel in a TDM system increases the total output data rate, thus requiring a new clock frequency and frame synchronization code. There is no resultant benefit in increased transmission signal strength when a TDM channel is deleted because the channel is simply replaced by a dummy data stream in order to maintain the same output data rate. Alternatively, channels can be added in FDM without disturbing the system. The removal of a channel in FDM results in improved performance because the extra power can be distributed equally among the remaining channels.
When FDM is chosen as the multiplexing scheme, the most important consideration, particularly in multi-channel, high bandwidth transmissions, is the generation of stable microwave subcarriers at the transmitter and local oscillator frequencies at the receiver. In the transmitter of an FDM system, it is very important that the frequencies of all subcarriers are extremely stable since adjacent-channel interference will occur if the frequency spacing between channels is not maintained under all operating conditions.
Conventional FDM systems transmitting a small number of channels generate the subcarrier frequencies with individual stable oscillators. This is adequate for a few channels, but for a system with a large number of channels this method is expensive, complex due to the DC wiring required, and inefficient in power consumption. At the receiver end, the channel selection is performed by tuning a voltage controlled oscillator. In a coherent MPSK system, the instability of the VCO frequency due to temperature drifts can cause loss of receiver synchronization and resultant loss of data.
In U.S. Pat. No. 4,726,011 granted to Ih et al., coherent optical carriers are generated at the transmission end by optical frequency shifting or injection-locking two or more lasers. In the optical frequency shifting approach, a reference beam from a single-mode laser diode is frequency shifted by the desired amount by sending it through a Traveling-Wave-Acousto-Optical-Modulator. The frequency-shifted beam is used to injection-lock a transmitting laser. This approach is not desired because it is inefficient and cumbersome to continually frequency-shift the reference laser beam when multiple carriers are needed.
An alternative way of generating optical carriers is to injection-lock a plurality of transmitting lasers to the sidebands of an FM modulated injection laser. When an injection laser is modulated near its resonant frequency, a large FM modulation will result. If the modulation index is properly adjusted, a large number of sidebands are generated. Although providing multiple carriers, this alternative has high power consumption because of the multiplicity of lasers used.
"Microwave Multiplexing Techniques for Wideband Lightwave Distribution Networks" by Olshansky et al., IEEE International Microwave Symposium Digest, 1988, promotes the use of multiplexed microwave subcarriers in providing wideband transmissions over optical fiber networks. The required microwave subcarriers are generated with a voltage controlled oscillator that is digitally modulated by FSK or frequency modulated with an analog signal. The instability of the VCO frequency due to temperature drifts may cause spectrum overlap among adjacent information channels.
"Subcarrier Multiplexing for Multiple-Access Lightwave Networks" by T.E. Darcie, IEEE Journal of Lightwave Technology, 1987, proposes a subcarrier multiplexing scheme where multiple channels are transmitted over a fiber optic link. The subcarriers, modulated by frequency shift keying (FSK), are generated by a voltage-controlled oscillator which introduces speed limitations because the oscillator cannot respond instantaneously to the voltage transitions. When data is applied to the VCO, some time is required to establish oscillations at each frequency and, unless the settling time of the VCO is extremely short, some undesired amplitude modulation will accompany the FSK signal. Furthermore, the shunt capacitance of the varactor diode can effectively low-pass filter the signal applied to the varactor and distort the FSK spectrum. At the receiving end of this system, a phase-locked loop FSK demodulator is used wherein a VCO tracks the subcarrier frequency of a desired channel.